This invention relates to a pressure control valve for a mechanically controlled power brake unit comprising essentially a control casing with at least an approximately cylindrical guide member extending out of the power brake unit, a valve closure member, a valve piston guided in the control casing, and a piston rod secured to the valve piston extending through the guide member and projecting therefrom, with the piston rod's one end being carried in a bore of the valve piston and being pivotable about that end.
From German Patent DE-OS No. 2,300,355 a power brake unit is known wherein the pressure control valve is arranged in a control casing. The control casing includes a guide member which is slidably carried in the vacuum casing. The front part of the control casing accommodates a valve piston and a valve closure member. Secured to the valve piston is a piston rod for the introduction of force, the rod extending through the guide member and projecting therefrom. In this arrangement, the piston rod has a spherical end carried in an opening of the valve piston.
The control casing is generally an inelastic plastic material which is well suited for injection molding techniques. Conventionally, an air filter enclosing the piston rod is arranged in the guide member. In addition to the air filter, a sound absorber may also be provided in the guide member.
Prior to the mounting of the power brake unit in an automotive vehicle, the piston rod is only carried on its spherical end and, consequently, pivotable about that end. The excursion of the piston rod is only limited by the edge of the guide member because the air filter and the sound absorber (if any) are highly elastic and, accordingly, exert no appreciable damping effect on the piston rod.
Before the power brake unit is mounted, for example, when in transit, the piston rod is subjected to lateral knocks which have to be taken up by the control casing. The guide member of the control casing is mostly thin-walled for reasons of weight and material and, accordingly, the degree of absorption of the lateral knocks acting on the piston rod is limited. Under increased knock impact, the control casing is frequently damaged. This is the case particularly with control casings consisting of two parts.